1. Field of the Invention
The present invention relates to a semiconductor acceleration sensor. More particularly, the present invention relates to a semiconductor acceleration sensor suitable for body control, engine control, airbag control, and the like of a vehicle.
2. Related Arts
Accurate detection of a comparatively low level (0 to +1 G) acceleration at a low frequency level (0 to 100 Hz) is necessary in an acceleration sensor for a vehicle. Herein, "1 G" is a unit of acceleration representing 9.8 m/sec.sup.2.
A piezoelectric type acceleration sensor utilizing a piezoelectric effect, an electromagnetic type acceleration sensor utilizing a differential transformer, and semiconductor types, such as a semiconductor strain gauge type or an electrostatic capacitance type acceleration sensor, are widely known for vehicle use. Of these, the semiconductor types are most capable of detecting low acceleration levels and low frequency levels with good accuracy and suitable for high volume production at low cost.
Additionally, the electrostatic capacitance type acceleration sensor is characterized by a high detection sensitivity in comparison with the strain gauge type.
The device disclosed in EP 0 369 352 B1, which is an electrostatic capacitance type acceleration sensor, is shown in FIG. 27. In FIG. 27, the detector portion of the electrostatic capacitance type acceleration sensor is composed of a directly bonded substrate, which is formed from three silicon substrates 201, 202, and 203 with thermal oxidation films 204 interposed therebetween. A silicon beam 205 and movable electrode 206 are formed on the silicon substrate 201 before bonding by means of etching. Moreover, fixed electrodes 207 and 208 composed of polycrystalline silicon are also formed on the silicon substrates 202 and 203 before bonding. The movable electrode 206, which functions as weight, is supported by means of the silicon beam 205, and in accordance with the vertical acceleration, which acts upon the movable electrode 206, the dimensions of the gaps between the movable electrode 206 and the fixed electrodes 207 and 208 change. As a result, the electrostatic capacitance of the gap portions changes in accordance with the acceleration acting on the detector portion, and detection of acceleration is accomplished by outputting this change to an external electronic circuit via a bonding pad 209.
However, the electrostatic capacitance type acceleration sensor requires sophisticated machining technology in order to machine the silicon substrate to 100 to 200 pm and form a thin beam, and along with this, production costs also increase.
In short, a total of three silicon substrates are required, including one silicon substrate to form a movable electrode and two silicon substrates to form fixed electrodes. As a result, it is difficult to reduce cost. Moreover, because mutual silicon substrates must be bonded via thermal oxidation films, thermal constraints are applied in terms of the process. Furthermore, because the detection of acceleration is performed by detecting changes in electrostatic capacitance, the electrode surface area, which forms the electrostatic capacitance, cannot be made smaller than the measurement lower limit thereof. As a result, smaller sizes could not be expected.